Broadband, freeform focusing micro optics for side-viewing imaging catheters

ABSTRACT

The disclosed embodiments relate to a system that implements a side-viewing imaging catheter. This system includes a catheter sheath enclosing an imaging core, wherein the imaging core presents an internal optical channel coupled to an optical element located at the distal end of the imaging core. The optical element includes an internal reflective surface that reflects and focuses light transmitted via the optical channel in a direction orthogonal to a rotational axis of the catheter toward a target location, and returns reflected light from the target location back through the optical channel. This internal reflective surface of the optical element is shaped to focus the light so that a resulting beam shape at the target location has a small cross section area and substantially equal axial and transaxial dimensions.

BACKGROUND Field

The disclosed embodiments generally relate to catheter-basedvascular-imaging systems. More specifically, the disclosed embodimentsrelate to the design of broadband, freeform micro optics forside-viewing imaging catheters.

Related Art

Imaging catheters and endoscopes are presently the focus of significantresearch effort, with diagnostic applications in the fields ofcardiovascular medicine, gastroenterology and pulmonology.Catheter-based optical imaging modalities share common characteristics,such as a fiber for delivery of light to tissue, distal optical elementsto direct and focus the beam, and a transparent sheath surrounding thedevice. A number of research groups have recently developed multimodaldevices, which include a morphological imaging modality, such as opticalcoherence tomography (OCT) or intravascular ultrasound (IVUS), combinedwith fluorescence or spectroscopic techniques that provide additionalcomposition information.

In the cardiovascular field, pulse sampling fluorescence lifetimeimaging (FLIm) provides great potential for characterizing tissuebiochemical information, because it is able to provide informationrelated to inflammation as well as changes in protein content. (See L.Marcu, J. A. Jo, Q. Fang, T. Papaioannou, T. Reil, J.-H. Qiao, J. D.Baker, J. A. Freischlag, and M. C. Fishbein, “Detection of rupture-proneatherosclerotic plaques by time-resolved laser-induced fluorescencespectroscopy,” Atherosclerosis 204, 156-164 (2009). This is achievedwithout the use of exogenous molecular probes, making it a goodcandidate for clinical translation. A FLIm implementation differs fromexisting modalities such as OCT in terms of the wavelength range as wellas the type of fiber used for light delivery and collection. Excitationis typically performed in the near ultraviolet (UV) range to match theabsorption range of many biological fluorophores. An excitationwavelength of 355 nm is commonly used due to the availability ofsub-nanosecond pulse diode pumped solid state lasers working at thiswavelength. Additionally, fluorescence photons need to be efficientlycollected leading to the use of large core multimode fibers (>100 μm).These additional constraints make two commonly used distal-end opticsdesigns, namely gradient index (GRIN) lenses and ball lenses poorlysuited for FLIm.

GRIN lenses are used for a variety of applications in the visible andnear-infrared range. However, commonly used dopants, such as sliverions, present strong absorption and autofluorescence at 355 nm.UV-compatible dopants such as lithium are available, but limits in theirrefractive index variations lead to a low numerical aperture (NA)(around 0.2). This low NA leads to relatively long (>3 mm) elements,which are unsuitable for vascular applications. Additional limitationsare strong chromatic aberrations and an optical index profile thatpresents a circular symmetry and, therefore, does not enable thecorrection of astigmatism introduced by the device sheath, which leadsto different focal distances in the axial and transaxial directions.

Another implementation of a distal-end optic comprises a fused balllens, where a short section of no-core fiber is spliced with the mainfiber and fused into a spheroid. The spheroid is polished to provide anangled facet that deflects the beam via total internal reflection. Withthis design, different radii can be obtained in the axial and transaxialdirections to correct for the astigmatism introduced by the devicesheath. A suitable geometry of the spheroid is achieved empirically byoptimizing the fusion process.

However, a ball lens termination for a large core multimode fiber (>100μm) requires the splicing of a larger no-core fiber to perform beamexpansion, which is itself fused into a sphere. Moreover, the longerheating period required for large spheres (>˜300 μm) leads to saggingduring the heating process, which causes an asymmetry of the geometry.Additionally, the use of the probe in liquids requires capping with ashort length of glass capillary because both total internal reflectionfrom the facet and focusing from the surface of the spheroid relies on aglass-to-air interface. Moreover, capping introduces additionalastigmatism and increases bulk and complexity.

Hence, what is needed is a distal-end side-viewing micro optic thatfacilitates excitation of a sample in the near UV range without thedrawbacks of existing micro optic designs.

SUMMARY

The disclosed embodiments relate to a system that implements aside-viewing imaging catheter. This system includes a catheter sheathenclosing an imaging core, wherein the imaging core presents an internaloptical channel coupled to an optical element located at the distal endof the imaging core. The optical element includes an internal reflectivesurface that reflects and focuses light transmitted via the opticalchannel in a direction orthogonal to a rotational axis of the cathetertoward a target location, and returns reflected light from the targetlocation back through the optical channel. This internal reflectivesurface of the optical element is shaped to focus the light so that aresulting beam shape at the target location has a small cross sectionarea and substantially equal axial and transaxial dimensions.

In some embodiments, a shape of the internal reflective surface isnumerically computed by optimizing a polynomial surface to minimize aradius of the beam shape at the target location.

In some embodiments, the internal reflective surface comprises anaspheric surface with additional polynomial aspheric terms.

In some embodiments, the internal reflective surface is fabricatedthrough direct-write laser machining in combination with a secondarysurface reflow operation.

In some embodiments, the internal reflective surface is shaped toreflect the light with a beam tilt in a forward axial direction.

In some embodiments, the optical element includes a reflective coatingto provide broadband reflectivity.

In some embodiments, the side-viewing imaging catheter is configured toperform ultraviolet (UV) imaging.

In some embodiments, the optical element is comprised of fused silica.

In some embodiments, the side-viewing imaging catheter is a multimodalcatheter, which supports both optical and ultrasonic imaging, whereinthe catheter tube additionally encloses an electrical channel, andwherein the probe additionally includes an ultrasonic transducer coupledto the electrical channel. This ultrasonic transducer is orientedorthogonally to a rotational axis of the catheter and is configured togenerate an ultrasonic acoustic signal and to return resulting echoinformation.

In some embodiments, the multimodal catheter supports both intravascularultrasound imaging and multispectral fluorescence-lifetime imagingmicroscopy.

The disclosed embodiments also relate to a process for manufacturing aside-viewing micro optic. During this process, one or more curvedsurfaces are created in a silica wafer, wherein the one or more curvedsurfaces have a geometry suitable to shape an internal optical beam byreflection. Next, a reflective coating is deposited on the silica waferto provide reflectivity, thereby converting the one or more curvedsurfaces into one or more internal freeform reflective surfaces.Finally, the silica wafer is cut to obtain one or more micro-opticelements, which are configured to receive an incoming optical beam alonga rotational axis of the catheter, wherein each micro-optic elementincludes a freeform reflective internal surface to reflect the incomingoptical beam in a substantially orthogonal direction from the opticalaxis toward a target location, wherein the internal reflective surfaceis shaped to focus the beam so that a resulting beam shape at the targetlocation has substantially equal axial and transaxial dimensions.

In some embodiments, the one or more curved surfaces are created usingdirect laser machining.

In some embodiments, the one or more curved surfaces are created using agrayscale lithography technique.

In some embodiments, the reflective coating is not deposited in caseswhere a total internal reflection with a surrounding medium issufficient to reflect the optical beam.

In some embodiments, creating the one or more curved surfaces involvescreating a microlens array comprising a large number of curved surfacesorganized in a rectangular pattern on the silica wafer.

In some embodiments, the cutting of the silica wafer is performed usinga dicing saw. This process involves: mounting the coated silica wafer ona silicon wafer using mounting media; cutting the microlens array alonga vertical direction of the microlens array to create individual stripsof microlenses; removing each microlens strip from the silicon wafer byheating the mounting media; positioning and securing each microlensstrip so that a side of the microlens strip is attached to a secondsilicon wafer using mounting media; using a dicing saw to cut eachmicrolens strip into individual microlenses; and performing a trimmingoperation on each microlens to obtain a specified length.

In some embodiments, performing the trimming operation on each microlensinvolves: removing the microlens from an underlying wafer; mounting themicrolens on a support such that the only part to be removed protrudesfrom the support; and polishing the microlens to a specified length.

In some embodiments, the cutting of the silica wafer is performed usinga laser.

In some embodiments, the cutting of the silica wafer is performed in asingle operation by tilting a cutting plane.

In some embodiments, the process also involves polishing an uppersurface of each microlens to limit scattering caused by roughnesscreated by the dicing process.

In some embodiments, while using the direct laser machining process tocreate the microlens array, the process ensures that a spacing betweenmicrolenses corresponds to a kerf width of a blade of the dicing saw.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A illustrates a reflective micro optic that facilitates both beamfocusing and reflection in accordance with disclosed embodiments.

FIG. 1B illustrates how a reflective micro optic is cut from a silicawafer in accordance with disclosed embodiments.

FIG. 2 presents a graph illustrating variations of a fluorescence signalas a function of distance for both a freeform optic and a flat prism inaccordance with disclosed embodiments.

FIG. 3 illustrates beam waist axial position and overall beam diameteras a function of distance in accordance with disclosed embodiments

FIG. 4 illustrates integration of a freeform optic into a FLIm-IVUSintravascular catheter in accordance with disclosed embodiments.

FIG. 5 presents a flowchart illustrating a process for fabricatingmicro-optic elements in accordance with disclosed embodiments.

FIG. 6 presents a flowchart illustrating a process for cutting amicrolens array to obtain microlenses in accordance with disclosedembodiments.

FIG. 7 presents a flowchart illustrating a process for trimming amicrolens in accordance with disclosed embodiments.

DETAILED DESCRIPTION

The following description is presented to enable any person skilled inthe art to make and use the present embodiments, and is provided in thecontext of a particular application and its requirements. Variousmodifications to the disclosed embodiments will be readily apparent tothose skilled in the art, and the general principles defined herein maybe applied to other embodiments and applications without departing fromthe spirit and scope of the present embodiments. Thus, the presentembodiments are not limited to the embodiments shown, but are to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

The data structures and code described in this detailed description aretypically stored on a computer-readable storage medium, which may be anydevice or medium that can store code and/or data for use by a computersystem. The computer-readable storage medium includes, but is notlimited to, volatile memory, non-volatile memory, magnetic and opticalstorage devices such as disk drives, magnetic tape, CDs (compact discs),DVDs (digital versatile discs or digital video discs), or other mediacapable of storing computer-readable media now known or later developed.

The methods and processes described in the detailed description sectioncan be embodied as code and/or data, which can be stored in acomputer-readable storage medium as described above. When a computersystem reads and executes the code and/or data stored on thecomputer-readable storage medium, the computer system performs themethods and processes embodied as data structures and code and storedwithin the computer-readable storage medium. Furthermore, the methodsand processes described below can be included in hardware modules. Forexample, the hardware modules can include, but are not limited to,application-specific integrated circuit (ASIC) chips, field-programmablegate arrays (FPGAs), and other programmable-logic devices now known orlater developed. When the hardware modules are activated, the hardwaremodules perform the methods and processes included within the hardwaremodules.

Technical Details

As is illustrated in FIG. 1A, the reflective micro optic, which wasdeveloped to address these challenges, comprises a fused silica element104 terminated by a curved surface 106 with a reflective coating thatperforms both beam focusing and reflection operations for an opticalbeam that originates from a multimode fiber 102. This design isinherently broadband because the optical beam is transmitted throughpure glass and reflections are not subject to chromatic aberrations. Animportant aspect of the performance of the proposed solution is to beable to fully specify the geometry of the reflective optical surface.This is because different radii of curvature are required to achieve anidentical focal plane in the axial and transaxial directions at a targetlocation, and astigmatisms introduced by other elements in the beam pathsuch as the device's sheath need to be corrected.

Direct-write laser machining of a fused silica substrate, such asLightForge (PowerPhotonic, Fife, UK), enables the manufacture of suchfreeform surfaces. (See Matthew Currie and Roy McBride, “Rapidmanufacture of freeform micro-optics for high power applications,” Proc.SPIE 8970, Laser 3D Manufacturing, 89700T (6 Mar. 2014); doi:10.1117/12.2040140.) In combination with a secondary surface reflowoperation, highly smooth optical surfaces with micrometer accuracy canbe manufactured over a clear aperture of 15×15 mm², provided the designpresents a sag of less than 50 μm and a slope of less than 45 degrees.Note that the machining depth is much smaller than the expected size ofthe optics, so to comply with these manufacturing constraints, thesimulated freeform element is positioned such that the freeform surfaceis tangential to a horizontal plane and therefore fully fits within a 50μm height as is illustrated in FIG. 1B. Subsequent cutting with a dicingsaw is used to separate the optics from the rest of the substrate.

A reflective surface, which is compatible with these constraints, wascomputed using Zemax (Radiant, Redmond, Wash.). The simulated modelincluded a fully filled 0.22 NA, 100 μm core multimode fiber and aUV-fused silica optical element having a 300×300 μm² cross section. Thefreeform reflective surface was defined as a conic aspheric surface withadditional polynomial aspheric terms, and was numerically optimized toprovide a 10-degree forward beam tilt and to minimize the beam RMSradius at 1.5 mm distance as is illustrated in FIG. 1A.

For efficient manufacturing, a microlens-like array replicating thefreeform surface to fully fill the clear aperture was created in Zemax.The pitch in both directions was set such that the clearance betweeneach of the micro optics corresponds to the width of the 100 μm widedicing blade (2.187-4A-30RU7-3, Thermocarbon, Inc., Casselberry, Fla.,USA). The design file was then converted for manufacturing into an arraythat defines the surface height data over a rectangular 10 μm pitch gridwith a Zemax macro provided by PowerPhotonic. After laser machining, aUV-enhanced aluminum coating was applied to provide broadbandreflectivity in the UV-visible range (Laseroptik, Garbsen, Germany).Individual micro optics were obtained by a two-step dicing operation,wherein the array was first mounted onto a 4″ silicon wafer withtemporary adhesive (Crystalbond 590, Electron Microscopy Sciences,Hatfield, Pa., USA), and then cuts were made between optics columns tocreate strips. Individual strips were then removed and mounted on theirside on a second wafer using temporary adhesive, and were then alignedand diced. The resulting individual optics were then mounted on asupport using temporary adhesive and polished to length using diamondlapping sheets (LFXD, Thorlabs, Newton, N.J., USA) before finalintegration into a device.

The freeform optics were assembled at the distal end of 100 μm corefiber optic (FVP100110125, Polymicro Technologies, Phoenix, Ariz., USA)using acrylate optical adhesive (OG603, Epoxy Technology, Inc.,Billerica, Mass., USA). This probe was connected to the FLIm system toperform the experimental characterization of the fluorescence signalintensity with respect to distance. (See D. Ma, J. Bec, D. R.Yankelevich, D. Gorpas, H. Fatakdawala, and L. Marcu, J. Biomed. Opt.19, 2014.) A 150 μm thick polystyrene sheet was used as a target. Thecombination of the 100 mm core fiber and the freeform optic was comparedwith existing 100 μm and 200 μm fibers terminated with flat prisms. SeeFIG. 2, which illustrates the resulting variations of the fluorescencesignal as a function of distance for the freeform optic as well as aflat prism. Note that for distances above 1 mm, the freeform optic incombination with a 100 μm core fiber provides results close to that of a200 μm core fiber despite a fourfold reduction in fiber cross section.Moreover, the collection efficiency obtained with the micro optics isalmost on par with the collection efficiency of a 200 μm fiber (83% at 1mm distance) despite the fourfold reduction in fiber cross section, andresults in a lower variation of signal as a function of distance: theratio of signal at a 3 mm distance to the maximum signal is 23.5% and11% respectively, leading to improvements in signal uniformity duringscanning.

The beam profile characterization was performed by coupling the fiberoptic to an extended light source (L9455-1, Hamamatsu Photonics,Hamamatsu, Japan) to achieve an overfilled launch condition. The beamprofile was measured using a 20× microscope objective and a 1,392×1,040CCD camera (CCE-B013-U, Mightex, Toronto, Canada), leading to a pixelsize of 1.047 μm. Note that multimode beams do not necessarily present aGaussian profile; therefore, the beam size was determined as the 80%encircled energy radius. A Zemax simulation of the beam profile wasperformed on the fiber and the freeform surface optics model describedpreviously with reference to FIG. 1A, and the 80% encircled energyradius was computed for different detector plane distances. Simulatedand experimental data, which are presented in FIG. 3, illustrate a verygood agreement on overall beam diameter as a function of distance aswell as beam waist axial position. Although the freeform surface wasoptimized to minimize the beam radius at 1.5 mm distance, the largefiber core to optics aperture ratio is such that the waist is locatedonly 600 μm from the surface of the element.

Finally, a series of FLIm-IVUS catheters integrating the freeform opticswere manufactured and highlighted additional key benefits of theproposed solution. See FIG. 4, which illustrates an integration of thefreeform optics 404 into a FLIm-IVUS intravascular catheter probe 402.This freeform optics 404 presents a large 300 μm aperture to enhancefluorescence signal collection and can be integrated into a compact 600μm diameter, 1.8 mm length probe 402, which also includes an ultrasonictransducer 406. The catheter probe and associated rotating componentsare surrounded by a non-rotating sheath 410, which is in direct contactwith patient. Because the beam is reflected internally on the freeformsurface, the element can be easily integrated into an imaging device bypotting the freeform optics 404 with adhesive, ensuring a high level ofprotection. Note that only the fused silica top surface is exposed tothe surrounding environment.

Process for Fabricating Micro-Optic Elements

FIG. 5 presents a flowchart illustrating a process for fabricatingmicro-optic elements in accordance with disclosed embodiments. First,the process creates one or more curved surfaces in a silica wafer,wherein the one or more curved surfaces have a geometry suitable toshape an internal optical beam by reflection (step 502). Next, theprocess deposits a reflective coating on the silica wafer to providereflectivity, thereby converting the one or more curved surfaces intoone or more internal freeform reflective surfaces (step 504). Finally,the process cuts the silica wafer to obtain one or more micro-opticelements, which are configured to receive an incoming optical beam alonga rotational axis of the catheter, wherein each micro-optic elementincludes a freeform reflective internal surface to reflect the incomingoptical beam in a substantially orthogonal direction from the opticalaxis toward a target location, and wherein the internal reflectivesurface is shaped to focus the beam so that a resulting beam shape atthe target location has a small cross section area and substantiallyequal axial and transaxial dimensions (step 506).

FIG. 6 presents a flowchart illustrating a process for cutting amicrolens array to obtain microlenses in accordance with disclosedembodiments. (This flowchart provides more details about the operationsinvolved in step 506 in the flowchart in FIG. 5.) First, the processmounts the coated silica wafer on a silicon wafer using mounting media(step 602). Then, the process cuts the microlens array along a verticaldirection of the microlens array to create individual strips ofmicrolenses (step 604). The process then removes each microlens stripfrom the silicon wafer by heating the mounting media (step 606). Theprocess then positions and secures each microlens strip so that a sideof the microlens strip is attached to a second silicon wafer usingmounting media (step 608) The process then uses a dicing saw to cut eachmicrolens strip into individual microlenses (step 610). Finally, theprocess performs a trimming operation on each microlens to obtain aspecified length (step 612).

FIG. 7 presents a flowchart illustrating a process for trimming amicrolens in accordance with disclosed embodiments. (This flowchartprovides more details about the operations involved in step 612 in theflowchart in FIG. 6.) First, the process removes the microlens from anunderlying wafer (step 702). Next, the process mounts the microlens on asupport such that the only part to be removed protrudes from the support(step 704). Finally, the process polishes the microlens to a specifiedlength (step 706).

CONCLUSION

A novel type of side-viewing optics based on a freeform reflectivesurface has been designed, manufactured and characterized. This newdesign addresses the shortcomings of ball lens and GRIN distal-endoptics and is ideally suited for fluorescence imaging due to its hightransmission and low autofluorescence in the UV range. The design of theelements was performed using standard optics simulation software wheresurrounding elements such as the catheter sheath are easily included. Byusing optimization techniques, the design of a freeform surface geometrythat best fulfills the design requirements and therefore maximizesimaging performances is straightforward. The direct-write lasermachining of the optics does not require the upfront investmentnecessary for alternative techniques, such as grayscale lithography ormolding, and can be easily outsourced. Moreover, the manufacturing stepsare performed with standard dicing and hand polishing equipment. The useof a temporary adhesive during the dicing and polishing steps is ideallysuited to the fixturing of miniature parts and protects the functionalsurface of the optics so no damage to any of the optics manufacturedwith this process occurred. By using the above technique, severalhundred optics can be manufactured from the same wafer and may include alarge variety of alternative designs, making the technique suitable forboth research and development and production. The reflective opticalcoating deposition is done on the wafer, so it is easy and verycost-effective because all optics of a batch can be coated at once.Using different types of reflective coating (metallic/dielectric) allowsthe optimization of the optics' performance for a specific wavelengthrange of operation, but good transmission and minimal chromaticaberrations can easily be achieved over the whole UV-to-NIR range. Thefreeform optics are currently attached to the fiber using adhesive socoupling losses may be higher than monolithic fused ball lens designs.Nonetheless, the ability to freely optimize the reflective surface, theinherently broadband design with minimal chromatic aberrations, and therobustness and ease of integration enabled by the reflection on aprotected internal surface make it an ideal component for intravascularFLIm. This broadband design may also lead to improvements in multimodaltechniques spanning a large wavelength range, such as the combination offluorescence and OCT.

Various modifications to the disclosed embodiments will be readilyapparent to those skilled in the art, and the general principles definedherein may be applied to other embodiments and applications withoutdeparting from the spirit and scope of the present invention. Thus, thepresent invention is not limited to the embodiments shown, but is to beaccorded the widest scope consistent with the principles and featuresdisclosed herein.

The foregoing descriptions of embodiments have been presented forpurposes of illustration and description only. They are not intended tobe exhaustive or to limit the present description to the formsdisclosed. Accordingly, many modifications and variations will beapparent to practitioners skilled in the art. Additionally, the abovedisclosure is not intended to limit the present description. The scopeof the present description is defined by the appended claims.

1. A side-viewing imaging catheter, comprising: a catheter sheathenclosing an imaging core; wherein the imaging core presents an internaloptical channel coupled to an optical element located at the distal endof the imaging core; wherein the optical element includes an internalreflective surface that reflects and focuses light transmitted via theoptical channel in a direction orthogonal to a rotational axis of thecatheter toward a target location, and returns reflected light from thetarget location back through the optical channel; and wherein theinternal reflective surface of the optical element is shaped to focusthe light so that a resulting beam shape at the target location has asmall cross section area and substantially equal axial and transaxialdimensions.
 2. The side-viewing imaging catheter of claim 1, wherein ashape of the internal reflective surface is numerically computed byoptimizing a polynomial surface to minimize a radius of the beam shapeat the target location.
 3. The side-viewing imaging catheter of claim 2,wherein the internal reflective surface comprises an aspheric surfacewith additional polynomial aspheric terms.
 4. The side-viewing imagingcatheter of claim 1, wherein the internal reflective surface isfabricated through direct-write laser machining in combination with asecondary surface reflow operation.
 5. The side-viewing imaging catheterof claim 1, wherein the internal reflective surface is fabricated usinga grayscale lithography technique.
 6. The side-viewing imaging catheterof claim 1, wherein the internal reflective surface is shaped to reflectthe light with a beam tilt in a forward axial direction.
 7. Theside-viewing imaging catheter of claim 1, wherein the optical elementincludes a reflective coating to provide broadband reflectivity.
 8. Theside-viewing imaging catheter of claim 1, wherein the side-viewingimaging catheter is configured to perform ultraviolet (UV) imaging. 9.The side-viewing imaging catheter of claim 1, wherein the opticalelement is comprised of fused silica.
 10. The side-viewing imagingcatheter of claim 1, wherein the side-viewing imaging catheter is amultimodal catheter, which supports both optical and ultrasonic imaging;wherein the catheter tube additionally encloses an electrical channel;and wherein the probe additionally includes an ultrasonic transducercoupled to the electrical channel; and wherein the ultrasonic transduceris oriented orthogonally to a rotational axis of the catheter and isconfigured to generate an ultrasonic acoustic signal and to returnresulting echo information.
 11. The side-viewing imaging catheter ofclaim 10, wherein the multimodal catheter supports both intravascularultrasound (IVUS) imaging and multispectral fluorescence-lifetimeimaging microscopy (FLIm).
 12. A micro optic for a side-viewing imagingcatheter, comprising: an optical element including an internalreflective surface that reflects and focuses light transmitted via anoptical channel in the catheter in a direction orthogonal to arotational axis of the catheter toward a target location, and returnsreflected light from the target location back through the opticalchannel; wherein the internal reflective surface of the optical elementis shaped to focus the light so that a resulting beam shape at thetarget location has substantially equal axial and transaxial dimensions.13. The micro optic of claim 12, wherein a shape of the internalreflective surface is numerically computed by optimizing a polynomialsurface to minimize a radius of the beam shape at the target location.14. The micro optic of claim 13, wherein the internal reflective surfacecomprises an aspheric surface with additional polynomial aspheric terms.15. The micro optic of claim 12, wherein the internal reflective surfaceis fabricated through direct-write laser machining in combination with asecondary surface reflow operation.
 16. The micro optic of claim 12,wherein the internal reflective surface is fabricated using a grayscalelithography technique.
 17. The micro optic of claim 12, wherein theinternal reflective surface is shaped to reflect the light with a beamtilt in a forward axial direction.
 18. The micro optic of claim 12,wherein the optical element includes a reflective coating to providebroadband reflectivity.
 19. The micro optic of claim 12, wherein theoptical element is configured to perform ultraviolet (UV) imaging. 20.The micro optic of claim 12, wherein the optical element is comprised offused silica.
 21. A method for manufacturing a side-viewing micro opticfor a catheter, comprising: creating one or more curved surfaces in asilica wafer, wherein the one or more curved surfaces have a geometrysuitable to shape an internal optical beam by reflection; depositing areflective coating on the silica wafer to provide reflectivity, therebyconverting the one or more curved surfaces into one or more internalfreeform reflective surfaces; and cutting the silica wafer to obtain oneor more micro-optic elements, which are configured to receive anincoming optical beam along a rotational axis of the catheter, whereineach micro-optic element includes a freeform reflective internal surfaceto reflect the incoming optical beam in a substantially orthogonaldirection from the optical axis toward a target location, and whereinthe internal reflective surface is shaped to focus the light so that aresulting beam shape at the target location has substantially equalaxial and transaxial dimensions.
 22. The method of claim 21, wherein theone or more curved surfaces are created using direct laser machining.23. The method of claim 21, wherein the one or more curved surfaces arecreated using a grayscale lithography technique.
 24. The method of claim21, wherein the reflective coating is not deposited in cases where atotal internal reflection with a surrounding medium is sufficient toreflect the optical beam.
 25. The method of claim 21, wherein creatingthe one or more curved surfaces involves creating a microlens arraycomprising a large number of curved surfaces organized in a rectangularpattern on the silica wafer.
 26. The method of claim 25, wherein thecutting of the silica wafer is performed using a dicing saw by: mountingthe coated silica wafer on a silicon wafer using mounting media; cuttingthe microlens array along a vertical direction of the microlens array tocreate individual strips of microlenses; removing each microlens stripfrom the silicon wafer by heating the mounting media; positioning andsecuring each microlens strip so that a side of the microlens strip isattached to a second silicon wafer using mounting media; using a dicingsaw to cut each microlens strip into individual microlenses; andperforming a trimming operation on each microlens.
 27. The method ofclaim 26, wherein performing the trimming operation on each microlenscomprises: removing the microlens from an underlying wafer; mounting themicrolens on a support such that the only part to be removed protrudesfrom the support; and polishing the microlens to a specified length. 28.The method of claim 21, wherein the cutting of the silica wafer isperformed using a laser.
 29. The method of claim 21, wherein the cuttingof the silica wafer is performed in a single operation by tilting acutting plane.
 30. The method of claim 21, further comprising polishingan upper surface of each microlens to limit scattering caused byroughness created by the dicing process.
 31. The method of claim 22,wherein while using the direct laser machining process to create themicrolens array, the method ensures that a spacing between microlensescorresponds to a kerf width of a blade of the dicing saw.